Intravenous anesthetic agents are typically short acting agents. The intravenous anesthetic agents are generally used in induction and maintenance phase of anesthesia. Based on the rapid distribution and metabolism of the anesthetic agents in patients' bodies, the anesthetic must be re-dosed frequently to ensure the anesthesia depth and the success of surgery. The control of the anesthesia amount is mainly based on the prediction of pharmacokinetic models. However, the pharmacokinetic models are not able to compensate the individual difference of each patient's physical characteristics, and may lead to determine a dose which may be an under-dose or overdose for the patient, either resulting in early wakeup during procedure or causing side effects. Therefore, precise and real-time detection of anesthetic concentration in plasma is greatly needed to improve the quality of anesthesia monitoring.
Different approaches are available to monitor patients under anesthesia procedures. These methods can be categorized into direct measurement of anesthetic drug concentration in blood and indirect measurement by monitoring a patient's conscious level, in addition to normal physiological parameters such as oxygen saturation, blood pressure, or heart rate. The anesthetic drugs may be detected in plasma or breath samples. Monitoring of anesthetic drug concentration in plasma or breath may provide better protection to patients than other conventional methods. The depth of anesthesia for a known concentration of drug in plasma is less variable; however, there is a significant interpatient variability in the drug concentration in plasma achieved with a known dose of anesthetic drug. The direct measurement of drug in plasma is invasive, time consuming and expensive. In contrast to direct method, an indirect breath based approach would be non-invasive, and provide continuous monitoring, faster response times and lower costs.
Therefore, the methods of monitoring a plasma concentration of intravenously delivered anesthetic drug by measuring the drug vapor concentration from exhaled breath are highly desirable.